Lila Stängel

No it's genetics . Exposed stems to LED light can turn red too.

Plants with red or pueplish leaf stems is common and nothing to worry about. Likely it's a genetic trait or a literal sun tan from the UV exposure. Violet main stems however, is very likely a Phosphorous deficiency. What causes the deficiency is the next step, but you don't have that issue. Relax dude your ok.

Some plants just have them... it can be a sign of low P, but you'd see other issues in the leaf as well as hindered growth. I see some link this to Mg, but i've never seen a leaf symptom chart that says this... I do find leaf symptom charts showing red/purple petioles for low-P, though. I've seen some youtube monkeys try excessively high Mg and it never had an impact. In my own experience, i've tried to boost P much higher and still had purple petioles same proportion of plants as always. I track my ppms of every molecule fed, so it's not a guess and is comparing to a known baseline of P with very little else changing.. .it is just a garden not a lab experiement, though. Variables do fluctuate and it'd be impossible to know for certain if that wasn't behind the small sample observations... though, i'm fairly confident. Even at 70+ ppm of P, it still happened. Even at lower levels i dont show any other potential signs of low p and plants grow fine. Canopies rarely show any damage, and even in senescence is fairly healthy.